1. Field of the Invention
The present invention relates to a solid oxide fuel cell having gas channel and particularly to a solid oxide fuel cell having gas channels which is implemented in an electrode support type or electrolyte support type of which four sides or opposite two sides of corners of a single cell are downwardly bent in an inverted U shape and have gas channels inside and/or outside the single cell.
2. Description of the Background Art
A fuel cell is an energy generation apparatus in which a fuel gas and air are injected to a anode and an cathode, respectively. An ionic conduction is performed through an electrolyte, and an electrochemical reaction is performed in the fuel cell. Electricity is continuously obtained provided that fuel and air are provided under certain conditions based on the characteristics of an electrode and electrolyte of the fuel cell as an electron conduction is proceeded through an external circuit.
The fuel cell is a power generation apparatus with relatively high efficiency and produces relatively small amount of pollutant. There are various operating temperatures, electrode materials and applications depending on the kinds of electrolytes.
In a Solid Oxide Fuel Cell (SOFC) referred to as an advanced fuel cell, an oxygen or hydrogen ionic conduction in a direction of a dense solid electrolyte layer is obtained based on a smooth flow of a reaction gas in a direction of an end cell electrode, an electrical contact with a separating plate, and a dense sealing between two kinds of reaction gases. In the SOFC, an electromotive force produced through an electrochemical reaction in the electrode layers is used for generating power.
Particularly, the SOFC uses a thermochemically stable metallic oxide as an electrolyte. A fuel gas such as hydrogen, methane, propane, butane, etc. may be used without reforming into a fuel electrode and air gas at an air electrode which are provided in the SOFC.
In well known solid oxide fuel cell materials, a mixture of Ni and YSZ cermet is used as a fuel electrode. One or more powder selected from a group consisting of ZrO2+8Y2O3 group, CeO2 group, Bi2O3 group, and perovskite group is used as an electrolyte. Also LaSrMnO3 (LSM) is used as an air electrode. Cr-5Fe-1Y2O3, Ni base metal, stainless steel, or LaSrCrO3 is used as a separating plate or an interconnect. A glass or glass-ceramic is used as a sealant. The above materials are stacked with each other and coupled with other peripherals. Therefore, the whole power generation system is constituted.
The above described single cell has a fuel electrode (cathode) in one side and an air electrode (anode) in the other side with respect to the electrolyte. Each electrode layer is designed to have a porous structure for implementing easier electrochemical reaction. The intermediate layer corresponding to an electrolyte is designed to have a dense structure in which a fuel gas and an oxide gas(air) are not communicated with each other.
A tube type and a flat plate type are developed in the SOFC based on the type of a single cell. The tube type is first developed, but there is a problem for actually adapting it on lower cost SOFC because a production method is not easy. Referring to FIG. 1A, in the flat plate type, a separating plate 8 is generally used, thus separating a fuel gas and an oxide gas, and the separated gas is provided to a single cell 4. The separating plate 8 has a small electrical resistance in order for the generated electricity to be conducted well.
Referring to FIG. 1B, when a system is manufactured in a stack structure, a flat type single cell is provided between the separating plates, and a sealant or a sealing glass is provided between the separating plates in order for two gases to flow through both channels of the separating plate not to be mixed with each other. A gas supply must be smoothly performed in the electrode layers at both sides of the single cell. Particularly, the remaining portions in which the single cell and the separating plate are not contacted must be sealed in an insulation layer structure or a plate shape formed of a certain material having certain sealing and insulation properties, for example, ceramic glass.
Metal and ceramic materials are used as a material of the separating plate. Particularly, the materials must have good electrical conductivity and good gastight sealing property. The materials must have an anti-oxidation property at a certain operation temperature of a solid oxide fuel cell in the range of, for example, from about 400. to about 1000° C. for the reasons that the reaction gas may be separated with respect to the separating plate and may be supplied to the single cell in a state that the hydrogen which is a fuel gas is separated in the side of the fuel electrode (upper surface in the separating plate), and the air which is an oxidizing agent is separated in the side of the air electrode of the single cell (lower surface in the separating plate). Therefore, it is advantageous to use the metallic separating plate for actually using the SOFC power generation system rather than using the ceramic material which is not produced easily and expensive. As the temperature and the operating time are increased, the surface of the metallic separating plate at the side of air electrode is oxidized, so that the performance of the stack is decreased. As a result, the durability of the system is decreased.
The metallic separating plate is widely used for developing the SOFC stack of an intermediate temperature (from about 500. to about 850° C.) because there are many advantages that the metallic separating plates may be produced in various types compared to the ceramic materials. However, in case of the SOFC stack, the thermal expansion coefficient of metallic separating plate has a large value 1.2 through 2 times the thermal expansion coefficient (about 10×10−6/° C.) of the single cell at which the SOFC operates, so that a larger thermal stress occurs as the temperature is changed from a room temperature to a high temperature or from a high temperature to a room temperature in a temperature change (namely, thermal cycle) when turning on or off the SOFC power generation apparatus. Recently, as a single cell having an excellent performance is developed based on the SOFC technology, the same performance as the conventional method is achieved at a relatively low temperature (intermediate temperature). Therefore, the metallic separating plate is more widely used compared to ceramic materials.
The separating plate formed of a metallic material or a ceramic material is generally formed in a channel structure for implementing a desired electrical contact with a smooth gas flow of injected reaction gas. Particularly, the ceramic material is formed based on a press compaction method. The production process is not complicated, but in the case of the metallic separating plate, it is produced based on a machining method, not based on the press compaction method. Therefore, it takes a long time for producing the gas channel which is a necessary structure in the fuel cell. In this case, the production cost of the separating plate and the whole production cost of the SOFC stack increase.
FIGS. 2A and 2B are showing the construction of a single cell (FIG. 2A) in which four sides or opposite two sides of corners of a rectangular single cell in U.S. patent application Ser. No. 09/522,284 (now U.S. Pat. No. 6,593,020, earlier filed by the same inventor as the present application) are bent in an inverted U shape and a stack construction (FIG. 2B) of a manifold type produced by using such a cell. The stack structure 11 includes a single cell 4, a gas channel 6, a channel support 7, separating plates 8 and 9, a porous insulating plate 10, a sealant groove 12, a first collector 14, a second collector 13, and as manifolds 15.
In the single cell and stack construction of FIGS. 2A and 2B, the fuel cell stack is produced in such a manner that the channel support 7 is provided between the grooves with the gas channel 6 on the separating plate 8, and the single cell 4 is coupled. The single cell 4 and the separating plate 8 are sealed using the porous insulation plate 10, namely, using a ceramic insulation felt and a sealing glass, for smoothing a sealed portion using the sealant groove 12 in the single cell 4 and the separating plate 8 and for preventing thermal stress. The above elements are coupled each other in the above sequence. The single cell and the above elements are finally stacked in vertical direction based on a necessary voltage for thereby producing the fuel cell stack.
It is possible to achieve an improved sealing condition by inserting sealant based on sufficient space (sealing groove) in which the portions needing a gastight sealing are separated, referring to FIGS. 2A and 2B, compared to the method of FIG. 1 in which the flat plate type single cell is simply used. Therefore, since the sealing function is improved, the entire stacks may be heated or cooled, so that a desired thermal cycle property is obtained. A certain stack structure having a good durability is implemented.
However, in the case that the stack of FIG. 2B is produced using the single cell of FIG. 2A, the channel structures of the channel support 7 and the separating plate 8 are additionally provided. In the above separating plate, the production cost of the separating plate may be increased due to the above channel structures, and the production cost of the SOFC stack is increased. Therefore, it is needed to provide the single cell 4 of FIG. 2B with a channel structure during the molding process, so that it is possible to directly use a separating plate formed in a simple plate shape which does not have a channel structure of a fuel electrode or air electrode during a stack production. Therefore, a relatively lower cost separating plate may be obtained when producing a SOFC stack.